A method to form an image by diffusion transfer using a silver salt such as silver halide or the like is publicly-known. Such a known method may comprise, for example, treating a photosensitive silver halide emulsion layer after being subjected to image exposure with an alkaline aqueous solution containing a developing agent, a solvent for a silver halide, and a film forming agent (a viscosity increasing agent) to reduce exposed silver halide grains with the developing agent to silver and, on the other hand, to change unexposed silver halide grains into transferable silver complex salts with the solvent for silver halide; diffusion transferring the silver complex salt to a layer (an image receiving layer) containing a precipitant for silver which is superposed on the above-mentioned emulsion layer by imbibition; and reducing the silver complex salt by use of the developing agent with the help of the precipitant for silver to form a silver image in the image receiving layer.
In carrying out the method, for example, a film unit is used which is prepared by combining a photosensitive element having a photosensitive silver halide emulsion layer disposed usually on a substrate, an image receiving element having an image receiving layer containing a precipitant for silver disposed on a substrate, and a processing element comprising a rupturable container housing an active alkaline aqueous solution containing a developing agent, a solvent for silver halide, and a film forming agent. After imagewise exposure of the emulsion layer of the photosensitive element, the photosensitive element and the image receiving element are superposed on each other with the emulsion layer facing the image receiving layer of the image receiving element, and then the two elements are run through a pair of rolls to rupture the processing element between the two rolls so that a viscous alkaline aqueous solution is formed between the elements. After that, the two elements are allowed to stand for a prescribed time before the image receiving element is stripped from the photosensitive element, and thus, a print having a desired image formed on the image receiving layer can be obtained.
In general, a method to form an image for diffusion transfer of silver salt has deffects that, when a developing time increases, the maximum density is lowered and at the same time, gradation of high density part becomes hard. That is, it has been found that when the developing time is increased, or the release time is increased in the case of release type photographic material, the silver amount in a part of the image receiving layer corresponding to the unexposed part of silver halide increases but a silver mirror reaction occurs markedly and surface reflection is caused remarkably, reducing the maximum density.